The present invention is concerned with a novel method of increasing the corrosion resistance and improving the organic protective coating characteristics of cold rolled flat steel. In one of its more specific variants, the invention is further concerned with a novel process for preparing container stock having a hardened coating of an organic protective coating thereon from cold rolled steel in the as rolled condition. The invention is also concerned with the various products produced by the method of the invention.
Cold reduced flat rolled mild steel products having thicknesses not greater than about 0.065 inch are usually produced by cold rolling hot rolled strip in coil form from a continuous hot strip mill. The sequence of operations involves uncoiling the hot rolled strip, passing the strip through a continuous pickler, rinsing, drying, oiling with a cold reduction lubricant which also protects against rusting, and recoiling. The resulting hot rolled strip is subsequently uncoiled and passed through a continuous cold reduction mill which has a plurality of stands with driven rolls. Modern continuous cold reduction mills operate at high strip speeds and often with heavy reductions in each stage and a large amount of heat is generated by frictional contact between the roll surfaces and the steel. The heat thus generated is removed by a system of flood lubrication in which streams or jets of an aqueous dispension of cold rolling oil are directed against the roll bodies and the surface of the steel. Following cold reduction to the desired gage, the cold rolled strip is coiled without removing the residual cold rolling oil and other surface contaminants such as iron oxide and dirt. Usually in container stock, the cold rolled strip is then uncoiled and cleaned to remove the residual rolling oil and surface contaminants. Thereafter the strip may be side trimmed and given any further treatments that are necessary or desirable for a given end use. Inasmuch as the freshly cold rolled and cleaned steel surface is very susceptible to rusting, often a protective coating is applied. The protective coating may be metallic in nature such as in the production of tin plate, or an organic protective coating may be applied such as in the production of lacquer or enamel coated blackplate.
The preparation of cold rolled flat steel having an organic protective coating thereon from cold rolled steel in the as rolled condition presents a number of problems. One serious problem is the above mentioned pronounced susceptibility to rusting while in transit to the customer and during storage awaiting use. Unless the as rolled steel is given a temporary protective coating, often rusting can not be controlled at an acceptable level while awaiting application of the final organic protective coating. The temporary protective coating increases the production cost substantially and usually adds very little, if anything, to the corrosion resistance of the final product. Thus, it would be possible to eliminate the temporary protective coating and reduce costs provided sufficient corrosion resistance to provide an adequate floor life could be imparted directly to the as rolled steel.
Another important disadvantage of the cold rolled steel lies in the organic coating characteristics of the initial as rolled surface. The residual cold rolling oil and perhaps other surface contaminants such as iron oxide and dirt cause the as rolled steel surface to have extremely poor organic coating characteristics. As a result, the as rolled steel surface can not be uniformly coated with a tightly adherent organic coating. The prior art has always taught that the organic coating characteristics can be improved only by carrying out a very efficient cleaning step. Usually the as rolled steel is passed through a strip cleaning line which employs a strong alkaline detergent solution to remove the rolling oil and other surface contaminants, and often the strip must be given an electrolytic treatment. The prior art has further taught that the cleaning step must be practiced prior to heating the cold reduced strip to a sufficiently elevated temperature such that the cold rolling lubricant would decompose, as the carbonaceous residue left on the strip surface was thought to be detrimental. The cleaning step adds substantially to the overall cost of processing the as rolled cold reduced strip and it would be desirable to eliminate this expense.
Prior to the present invention, an entirely satisfactory process was not available to the industry for overcoming the above mentioned difficulties with as rolled cold rolled steel to be given an organic protective coating. As a result, up until the present invention the art has followed the traditional practices of carrying out an expensive cleaning step to remove residual rolling oil and other surface contaminants and applying a temporary protective coating designed to increase floor life prior to applying the final organic protective coating.
It is an object of the present invention to provide a method of increasing the corrosion resistance of cold rolled flat steel in the as rolled condition.
It is a further object to provide a method of improving the protective organic coating characteristics of cold rolled flat steel in the as rolled condition which has iron oxide and a thermally degradable rolling oil on the surface thereof.
It is still a further object to provide a method of preparing container stock coated with a protective organic coating from cold rolled steel in the as rolled condition.
It is still a further object to provide cold rolled flat steel products prepared in accordance with the method of the invention.
Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the Examples.